The invention relates to a circuit arrangement for feeding a discharge lamp, comprising
input terminals for the connection to a supply voltage source,
switching means coupled to the input terminals for generating a high-frequency current from a supply voltage supplied by the supply voltage source,
a control circuit coupled to the switching means for rendering the switching means high-frequency conducting and non-conducting,
a temperature-dependent impedance for preheating electrodes of the discharge lamp.
The invention also relates to a compact lamp.
A circuit arrangement as mentioned in the opening paragraph is known from U.S. Pat. No. 4,935,672. In the known circuit arrangement, the switching means form part of an inverter of the half-bridge type. A load branch, which during operation contains the lamp, is coupled to this half bridge. The temperature-dependent impedance is formed by a PTC, which shunts the lamp and is connected in series with the electrodes of the lamp. When the circuit arrangement is in operation, the switching means generate a high-frequency current through the load branch. Immediately after the circuit has been put into operation, the temperature of the PTC is relatively low. As a result, also the impedance of the PTC is relatively low. This causes a current with a relatively high amplitude to flow through the electrodes of the lamp, and the voltage across the lamp, which is equal to the voltage across the PTC, to be relatively low. In this stage of operation of the lamp, the electrodes of the lamp are preheated. Since the PTC carries a current, the temperature of the PTC increases and hence also the impedance of the PTC. As the impedance of the PTC increases, the amplitude of the current through the electrodes decreases, and the amplitude of the voltage across the lamp increases to a value at which the lamp ignites. The presence of the PTC in the known circuit arrangement thus causes the electrodes of the lamp to be preheated before the lamp is ignited. A drawback of the known circuit arrangement resides in that the PTC is a relatively expensive component which must be added to the circuit arrangement for preheating the electrodes. In addition, the PTC also carries a current during normal operation of the lamp, so that a certain amount of power is dissipated in the PTC.
It is an object of the invention to provide a circuit arrangement for feeding a lamp, which circuit arrangement can also be used to heat the electrodes of the lamp before the lamp is ignited, which circuit arrangement is, in addition, relatively inexpensive and has a relatively high efficacy.
To achieve this, a circuit arrangement of the type mentioned in the opening paragraph is characterized in accordance with the invention in that the temperature-dependent impedance comprises, during operation of the lamp, one of the electrodes of the lamp and forms part of the control circuit.
As the temperature-dependent impedance comprises an electrode of the lamp, the circuit arrangement is relatively inexpensive. In addition, the load branch of the circuit arrangement does not comprise components which, during normal operation, do not fulfill a function but do dissipate power. As a result, the efficiency of a circuit arrangement in accordance with the invention is relatively high.
Good results have been obtained with embodiments of a circuit arrangement in accordance with the invention, wherein the switching means comprise a series arrangement of two switching elements.
A circuit arrangement in accordance with the invention can very suitably be used in the electronic ballast of a compact lamp comprising
a light source provided with a gastight lamp vessel which allows passage of visible light,
a housing which is secured to the light source and provided with a lamp cap,
an electronic ballast which is electrically connected to the light source in order to feed the light source, which electronic ballast is situated in a space which is surrounded by the housing.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.